Audio signal reproduction

ABSTRACT

An amplifier stage uses a loaded transistor amplifier circuit including a load that causes greater second order harmonic distortion energy than third order harmonic distortion energy to be produced in said loaded transistor amplifier circuit for amplifying a source audio signal to produce an audio output signal. The spectrum of the fundamental orders of harmonic distortion is adjusted to improve perceived sound quality or listening enjoyment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional patentapplication 63/217,846 filed Jul. 2, 2021, the contents of which arehereby incorporated by reference

TECHNICAL FIELD

This patent application relates to the field of audio signalreproduction.

BACKGROUND

An audio amplifier's Total Harmonic Distortion (THD) specification isoften used as a decisive measure of quality when comparing one amplifierto another. THD defines the combination of all orders of harmonicdistortion. When an audio amplifier circuit amplifies a single frequencytone, say at a frequency of 1 kHz, the output signal will typicallycontain the amplified signal at 1 kHz with distortion energy atdifferent orders, namely 2 kHz, 3 kHz, 4 kHz, 5 kHz, etc. While theharmonic distortion energy generally decreases with increasing order,many amplifiers reduce THD by using an amplifier circuit design thatreduces the second order that contains the greatest amount of energy.These types of amplifiers have relatively greater odd orders of harmonicdistortion than even orders.

Amplifiers with lower THD specification are typically regarded as best.Various techniques are employed to further reduce an amplifier'sfundamental THD measurement thereby providing a lower THD specificationfor publication. Depending on the magnitude of the techniques employedto lower the THD of an amplifier, it is questionable whether acorresponding improvement in subjective quality is ever achieved. Infact, quite often the subjective quality actually degrades as a resultof these techniques. THD specifications as a measure of subjective soundquality therefore are unreliable.

Vacuum tube amplifiers with their predominantly even order THD spectrumtypically sound warm and smooth. Solid-state amplifiers with theirpredominantly odd order THD spectrum sound less so by comparison. Hybridamplifiers comprised of both vacuum tube and solid-state technology havea mixture of both tube and solid-state orders of harmonic distortion andtherefore have combined subjective sound quality of both vacuum tubesand solid-state. It is very subjective whether vacuum tube, solid-stateor hybrid sound is best but the harmonic distortion spectrum of eachtechnology is very unique and identifiable.

Harmonic distortions spectrums associated with each stage of production,post-production and reproduction of an audio media file combine to forma complex spectrum of harmonic distortion at the final output.Production of the master media file includes harmonic distortionsassociated with the recording equipment. Post-production includes theproduction harmonic distortions as well as those associated with themixing and manufacturing of the final analogue or digital media file.Reproduction of the media file includes all of the aforementionedharmonic distortions but also those associated with the equipmentutilized for playback including the transducers utilized for creatingthe final acoustic interface. Wired or wireless transmission technologyin any of the stages also contributes unique harmonic distortions to theoverall compound spectrum of harmonic distortion.

The THD formula accounts for the quantum of the overall compoundspectrum of harmonic distortion but not the nuances of the structuralrelationship between one order of harmonic distortion and another. Toproperly understand the perceived quality of an audio signal, one needsto know the quantum of each order of harmonic distortion in the spectrumcomprising the THD. Once this structural relationship is understood andpresented along with the THD measurement, a more meaningful and reliableindication of perceived quality can be made.

SUMMARY

The sound quality of a reproduced audio signal is a combined function ofthe harmonic distortion of the audio signal, the employed transmissiontechnology, the circuit topology of the amplifier including thecumulative effect from all of the amplifiers in the chain ofreproduction. Distortions formed and recorded in the original masterfile combine with the harmonic distortion of the transmission technologyand amplifying circuits to produce a compound harmonic distortionspectrum. These compound harmonic distortions contribute to the overalldistortion of the system and are responsible for the overall sonicsignature and the resultant perceived quality of sound. Thesedistortions can be measured as individual orders of harmonic distortionwhich are multiples of the audio signal being reproduced. Typically, the2nd, 3rd, 4th & 5th harmonics of distortion of an audio signalcontribute the most to the sonic signature of an amplifier but there arecases where higher orders of distortion above the 5th order can as well.

Different technologies exhibit different spectrums. For example, vacuumtube amplifiers are dominant in even order harmonics but also havediminished odd order harmonics. Solid-state amplifiers are dominant inodd order harmonics but also have diminished even order harmonics.Amplifiers with predominate even order harmonics are typically smoothersounding subjectively and more natural sounding as a result. Solid-stateamplifiers with predominate odd order harmonics are less so. Thecomposition of the harmonic distortion spectrum requires both even andodd harmonic orders to be present and in a structured order to achievewhat is considered balanced sound.

A balanced spectrum of harmonic distortion adheres to certain principlesbut is also somewhat subjective and personal as it depends on thephysiology of one's hearing system and the normal age-related changesthat occur over time. Having the ability to shape the harmonicdistortion spectrum allows subjective changes based on the physiology ofthe target listener but also for amplifier designs to have a perceivedsonic character that is not limited strictly to the technology of theamplifier as it is today. As an example, a vacuum tube amplifier'sharmonics of distortion can be made to sound like a solid-stateamplifier and vice versa. The benefit of this is very compelling as thereliability and energy efficiency of a solid-state amplifier can nowperform like a vacuum tube amplifier.

Applicant's embodiment called a Harmonic Balancing Amplifier (HBA) is anadaptive harmonic spectrum amplifier used to adjust the spectrum of thefundamental orders of harmonic distortion. Input impedance loadingswithin the circuit produce an output impedance load on the audio signalwhich provides various spectrums of harmonic distortion having acompound effect on the overall spectrum. A single HBA circuit canproduce an adjustable harmonic spectrum and a multiple HBA circuit canprovide additional complex adjustability.

HBAs can be cascaded or paralleled using multiple circuits to furthercustomize a best fit harmonic distortion spectrum. The output gain of asingle or multiple HBA can be configured as either an audio source,preamplifier or power amplifier or in unity gain as part of an amplifieror standalone amplifier to provide preconditioning or post conditioningfor a system and audio signal.

A further embodiment provides overall harmonic reduction to the audiosignal which lowers all orders of harmonic distortion to allow greaterflexibility in using the adjustability of single or multiple HBAcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments can be better understood by way of the appendeddrawings and discussion.

FIG. 1 illustrates the typical Fast Fourier Transform (FFT) of theoutput signal of an ordinary Class AB solid-state audio signal amplifierusing the Federal Trade Commission (FTC) uniform test tone criterion of1 kHz. The even harmonic distortions are very low in comparison to theodd orders of harmonic distortions. This spectrum is typical of asolid-state amplifier which is often characterized as more fatiguing andless intelligible than a vacuum tube amplifier.

FIG. 2 illustrates the typical FFT of the output signal of an ordinaryClass AB vacuum tube audio signal amplifier using the same FTC uniformtest tone criterion of 1 kHz. The even order harmonic distortions arehigher in comparison to the odd orders of harmonic distortions and aretherefore deemed dominant. Dominant even order harmonics, compared toodd orders, with diminishing amplitudes as the orders increase, providea sound that is characterized as less fatiguing and more intelligiblethan solid-state technology.

FIG. 3 is a schematic circuit diagram of the Applicant's HBA embodiment.A variable input load VR1/C2 impedance inversely varies the outputimpedance load on transistors Q1/Q2 with a gain of approximately 20times or 26 dB thereby altering the orders of distortion of the audiosignal output.

FIG. 4A is an illustrative graph of distortion measurements based on thecircuit mentioned in FIG. 3 above using a single fundamental testfrequency input signal 10 with a variable load VR1/C2 impedance set tohigh thereby creating a very low output impedance on transistors Q1/Q2.At the output, the composition of the orders of harmonic distortions h2,h3 and h4 demonstrate a sequential downward trend for both even and oddorder harmonics as the orders increase.

FIG. 4B is an illustrative graph of distortion measurements based on thecircuit mentioned in FIG. 3 above using a single fundamental testfrequency input signal 10 with variable load VR1/C2 impedance set tomoderate (approximately equal to the source impedance of the amplifier)thereby creating a moderate impedance on transistors Q1/Q2. At theoutput, the composition of the orders of harmonic distortions h2, h3 andh4 demonstrate dominant odd 3rd order harmonics with reduced even orderharmonics.

FIG. 4C is an illustrative graph of distortion measurements based on thecircuit mentioned in FIG. 3 above using a single fundamental testfrequency input signal 10 with variable load VR1/C2 impedance setslightly higher than the source impedance thereby creating a slightlyless than moderate impedance on transistors Q1/Q2. At the output, thecomposition of the orders of harmonic distortions h2, h3 and h4demonstrate dominant even order harmonics with reduced odd orderharmonics and decreasing amplitudes as the orders of harmonicdistortions increase. The THD in FIG. 4C is greater than that of FIG.4B.

FIG. 4D is an illustrative graph of distortion measurements based on thecircuit mentioned in FIG. 3 above using a single fundamental testfrequency input signal 10 with VR1/C2 impedance set to very low creatinga very high loading effect on transistors Q1/Q2. At the output, thecomposition of the orders of harmonic distortions h2, h3 and h4demonstrate dominant odd order harmonics with decreasing amplitude forboth odd and even as the orders increase. The THD in FIG. 4D is greaterthan the THD in FIG. 4C.

FIGS. 4E to 4G are plots of measured THD residual for a power amplifierprototype showing minimum THD of a preamplifier, optimized THD of apreamplifier and overall amplifier THD resulting from minimumpreamplifier THD, respectively.

FIG. 5A is a schematic diagram showing one of the Applicant's alternateembodiments in which multiple amplifier circuits of the type identifiedin FIG. 3 are cascaded and arranged and adjusted to form a singlecompound audio amplifier circuit for the purposes of creatingcustomization in the harmonic spectrum beyond the variabilitydemonstrated in FIGS. 4A,B,C & D.

FIG. 5B is a schematic diagram showing another of the Applicant'salternate embodiments in which multiple amplifier circuits of the typeidentified in FIG. 3 are paralleled, arranged and adjusted to form asingle compound audio amplifier circuit for the purposes of creatingcustomization beyond FIGS. 4A,B,C & D and FIG. 5A options necessary tobalance the overall spectrum of harmonic distortion.

FIG. 6A is a schematic diagram that demonstrates alternate embodimentsusing both pre-compensation and post-compensation capabilities of theApplicant's design. Using multiple audio amplifier apparatus including alow distortion audio source, a pre-amplifier, a power amplifier and atransducer such as a loudspeaker, an alternate embodiment for astand-alone piece of equipment designed to complement the overalldistortion spectrum of the entire system whereby the output at the poweramplifier outputs has balanced orders of distortion.

FIG. 6B is a graph of the fundamental distortion of a typicalsolid-state preamplifier reproducing a 1 kHz sine wave input signal.

FIG. 6C is a graph of the fundamental distortion of the sameaforementioned solid-state preamplifier depicted in FIG. 6B.

FIG. 6D is a graph of the fundamental distortion of a typical 100-Watttotal power off-the-shelf solid-state power amplifier.

FIG. 6E is the compound effect of all harmonic distortions as measuredat the power amplifier output.

FIG. 7 is a schematic circuit diagram of a more complex circuit using anadjustable current source and a harmonic reduction circuit as a furtherembodiment of the Applicant's design.

FIG. 8 is a schematic diagram of a digital audio playback apparatusincluding an audio signal processor that creates and combines a desiredorder of harmonic distortion to the audio track. In this simplifiedexample, a conventional solid-state power amplifier with dominant oddorder harmonics of distortion can be balanced to have an output similarto the dominant even order harmonic spectrum in FIG. 2 to provide asound that is characterized as less fatiguing and more intelligible.With this example, a solid-state amplifier now has a harmonic distortionspectrum of a vacuum tube amplifier.

DETAILED DESCRIPTION

FIG. 1 demonstrates how harmonic distortions behave in a conventionalsolid-state power audio amplifier using a single uniform 1 kHz testtone. Even order harmonic distortions are created at 2, 4, 6 and 8 kHzwith odd order harmonic distortions at 3, 5, 7 and 9 kHz. The odd orderharmonics are dominant and uniform compared with the lesser even ordersof harmonic distortion. With a 35 dB output, the odd orders of harmonicdistortion all measure in the approximate range of −17 dB. The evenorders of harmonic distortion range from −55 dB at the second order to−45 dB at the eighth order showing a gradual increase of energy as theorders increase. This spectrum is typical of a solid-state amplifierwhich is often characterized as more fatiguing and less intelligiblethan a vacuum tube amplifier which has a dominant even order spectrumand diminished even and odd amplitudes as the orders increase with eachodd order less than the preceding even order.

FIG. 2 shows for comparison a vacuum tube based audio power amplifierusing a single uniform 1 kHz test tone. Even order harmonic distortionsare created at 2, 4, 6 and 8 kHz with odd order harmonic distortions at3, 5, 7 and 9 kHz. The even order harmonics are dominant with each oddorder less than the preceding even order. With a 30 dB output, the evenorders diminish with each successive even order and measureapproximately between −2 dB at the 2nd order to −30 dB at the 8th. Eachsuccessive odd order harmonic is less than the preceding even order andmeasure approximately between −16 dB at the 3rd order to −24 dB at the9th. The spectrum of odd orders diminishes with each successive orderfrom the 3rd through to the 7th order and increases slightly only at the9th which typically is not discernible.

The overall harmonic distortion of FIG. 2 is higher than that of FIG. 1but conforms to a spectrum that is considered balanced whereas FIG. 1does not. As a result, the perceived sonic quality of FIG. 2 is betterproviding less fatigue and greater intelligibility in spite of theoverall higher harmonic distortion.

FIG. 3 is a schematic circuit diagram of an audio amplifier circuit witha gain of approximately 20 times or 26 dB. The amplifier has a firststage comprising a transistor Q1 whose supply is provided by Q2. Theamplifier has an output stage comprising a transistor Q3. The gain ofthe circuit can be varied without affecting the spectrum of harmonicdistortion. The gain can also be close to unity when the function of theamplifier is to simply alter the spectrum of harmonic distortions in anaudio signal.

The audio amplifier circuit of FIG. 3 can alter the output impedance byadjusting the input impedance using a combination of VR1/C2. This loadcan be either fixed or adjustable using a variable resistor at VR1and/or a variable capacitor at the location of capacitor C2 forattaining different harmonic distortions necessary for balancing theoverall harmonic distortion spectrum of an amplifier.

By adapting the values of VR1/C2 in the circuit 15 of FIG. 3 and byusing a frequency dependent load circuit, it is also possible togenerate various frequency dependent distortion spectrum profiles. As anexample, such configurations could be used to change the distortionprofile at medium frequencies leaving the low and high frequencydistortions unchanged.

When cascading two or more circuits in circuit 15 of FIG. 3 with orwithout gain, it is possible to change distortion profiles by additionor cancellation of even or odd orders of harmonic distortion.

In FIG. 4A, the high input impedance VR1/C2 provides a low outputimpedance on transistors Q1/Q2 providing a sequential downward trendingof both even and odd harmonics as the orders increase. This providesdiminishing orders of harmonic content for balancing of the overallharmonic distortion spectrum where both even and odd orders arerequired.

In FIG. 4B, the input impedance VR1/C2 is approximately the same as thesource impedance resulting in a moderate output impedance on transistorsQ1/Q2 providing a dominant third order harmonic distortion with very loweven orders of harmonic distortion. This has limited application otherthan where 3rd order harmonics need to be added specifically to balancethe spectrum.

In FIG. 4C, the input impedance VR1/C2 is slightly higher than thesource impedance resulting in less than moderate output impedance ontransistors Q1/Q2 providing dominant even order harmonics with reducedodd order harmonics and decreasing amplitudes as the orders of harmonicsdistortions increase. This provides additional content for an alreadybalanced spectrum or where additional even orders of harmonics arenecessary to balance the spectrum.

In FIG. 4D, the low input impedance VR1/C2 provides a high outputimpedance on transistors Q1/Q2 providing dominant odd order harmonicswith decreasing amplitude for both odd and even as the orders increase.At the extreme this condition will overload the audio signal with toomuch harmonic distortion so the application is limited to addingincreased orders of both even and odd orders of harmonic distortion whenconsidered necessary to balance the harmonic distortion spectrum.

It is important to note that FIGS. 4A, 4B, 4C and 4D are simplifiedexamples and their harmonic behaviour continues beyond h4. It is alsoimportant to note that FIGS. 4A, 4B, 4C and 4D are examples of thevariability of the HBA circuit which is not strictly limited to thesefour loading examples and could, therefore, have any load necessary toachieve the spectrum of harmonic distortion desired.

FIGS. 4E, 4F and 4G show measured THD from a test power amplifiercircuit having a preamplifier circuit and a 100-watt power amplifiercircuit. The preamplifier has a sine wave generator with a THD of0.00002% at 2 Vrms into 30 kOhm load, and internal impedance of 20 Ohm.The preamplifier circuit power supply voltage is +/−20 VDC. The outputvoltage is 7.5 Vrms, the load is 15 kOhm, the input impedance is 30kOhm, the gain is 20 dB and the bandwidth is 300 kHz. The poweramplifier circuit is 100 watts at 4 Ohms for a power output of 25 Wrmsinto a 4 Ohm load. As shown in FIG. 4E, the measured preamplifier THD is0.00892% with the distortion at 3 kHz greater than at 2 kHz. In FIG. 4F,the optimized preamplifier (as in FIG. 3 ) can provide a THD of0.10874%, namely greater than the minimum THD of the preamplifier,however, with the harmonic distortion having decreasing energy withorder 2, 3, 4 and 5. The resulting power amplifier output shown in FIG.4G maintains this harmonic distortion having decreasing energy withorder 2, 3, 4 and 5.

As will be appreciated, an audio power amplifier circuit with an outputcharacteristic similar to FIG. 4B combined with a pre-amplifier circuitsimilar to FIG. 4C will provide a combined effect similar to 4A and willhave a perceived quality with less fatigue and greater intelligibilitythan combining a power amplifier and a pre-amplifier that are bothsimilar to FIG. 4B. FIGS. 5A and 5B shows two different topologyadaptions of FIG. 3 to achieve this.

FIG. 5A is a schematic diagram showing other embodiments in whichamplifiers 15, 15′ and 15″ are cascaded and arranged by using differentgain configurations. By giving different harmonic distortioncharacteristics to each of the amplifiers 15, 15′ and 15″ in the series,the spectrum of the harmonic distortion can be balanced using morecomplex configurations. The circuit of FIG. 5A can operate as either apower amplifier or pre-amplifier by varying the gain design of each ofthe stages.

FIG. 5B is also a schematic diagram showing an alternate embodiment inwhich amplifiers 15″ are arranged in parallel with adjustable gainlevels and a vector sum circuit to generate various distortion spectrumprofiles necessary for balancing the overall spectrum of harmonicdistortion. The circuit of FIG. 5B can operate as either a poweramplifier or pre-amplifier by varying the gain design of each of thestages.

FIG. 6A demonstrates pre-compensation and post-compensation capabilitiesof the Applicant's design which can be used whether the source audiosignal is transmitted via wired or wireless transmission. Consideringthe orders of harmonic distortion of the transmission medium in additionto the receiver and amplifiers at the receiving end allows apre-compensated spectrum to be transmitted resulting in balanced ordersof distortion at the speaker outputs of the receiving amplifier. Postcompensation alternatively can be implemented to provide for a similarbalanced order of harmonic spectrum at the speaker outputs.

FIG. 6A also demonstrates pre-compensation for the orders of harmonicdistortion at the transducer level using the Applicant's design.Considering the orders of harmonic distortion of the transducer inaddition to the other amplifiers in the overall system allows apre-compensated spectrum to be transmitted to the transducer resultingin an acoustic output that conforms to a balanced spectrum ofdistortion.

FIG. 6A can be further demonstrated using actual test resultmeasurements as provided in FIGS. 6B, 6C, 6D and 6E.

FIG. 6B is a graph of the fundamental distortion of a typicalsolid-state preamplifier reproducing a 1 kHz sine wave input signal withnegligible 0.00002% THD at 2 Volts RMS into a 30 kOhm load and apreamplifier internal impedance of 20 Ohm. The circuit power supply is+/−20 Volts DC with an output voltage of 7.5 Volts RMS into a 15 kOhmload. The gain of the circuit is 20 dB and the bandwidth of thepreamplifier is 300 kHz. Similar to the solid-state amplifier distortiongraph depicted earlier in FIG. 1 , FIG. 6B demonstrates an unbalancedspectrum of distortion common to most solid-state preamplifiers.

FIG. 6C is a graph of the fundamental distortion of the sameaforementioned solid-state preamplifier depicted in FIG. 6B but with theApplicant's embodiment adjusted to balance the overall output of thesystem. Providing compensation at the preamplifier also providespre-compensation for the power amplifier which results in a balancedoutput at the speaker terminals. Similar to the FIG. 4A effect, thedistortion has been altered from a dominant odd order spectrum to asequential downward trending spectrum of both even and odd orderharmonics as the orders increase.

FIG. 6D is a graph of the fundamental distortion of a typical 100-Watttotal power off-the-shelf solid-state power amplifier which is designedto produce an output of 25 Watts RMS power into a 4 Ohm load. The evenorder harmonic distortions are not dominant in comparison to therespective odd orders of harmonic distortions. Though typical for asolid-state amplifier, the orders of harmonic distortion are consideredunbalanced as are most solid-state amplifiers.

FIG. 6E is the compound effect of all harmonic distortions as measuredat the power amplifier output having the Applicant's embodiment forbalancing the entire compound system located at the preamplifier. Thesystem's combined orders of distortion represent a more balancedspectrum overall as measured at the power amplifier output. Although allof the equipment is solid-state, the compound spectrum of the orders ofdistortion measure closer to how a vacuum tube system would measure. Thesubjective performance of this compound system would be less fatiguingand more intelligible than without the harmonic adjustment provided inFIG. 6C.

FIG. 7 illustrates an example of the Applicant's HBA circuit embodimentwith a provision for harmonic distortion profile adjustment which canreduce the overall harmonic content. The topology is a J-FET/bipolarcascade stage with Q9 and Q8, biasing is provided by a J-FET Q10 workingas a constant current source and biased by R4 and R14. Q8 base potentialis produced by R3 and R5 and stabilized by C4. At the gate circuit of Q9there is a biasing return resistor R11 and a network composed by R2,R12, and C3. This network is a local negative feedback around Q8 and Q9,and it provides a gain reduction and stabilization so as to change thecurrent/voltage (I/V) curves of the circuit. The change in I/V curvesforces the circuit to produce a predominant even harmonic distortionprofile by bending its transfer function.

The collector load resistor R1 of Q8 and bipolar transistors Q1, Q2, Q5,Q6 and Q7 form the output circuit of the amplifier. The transistorstring in the output circuit is in fact an active load. By using the“Harmonics reduction” selector as shown in FIG. 8 , it is possible toreduce progressively the amount of harmonic distortion by successivebending of the transfer function in a way to adjust the amount ofharmonic residuals produced by the circuit.

By selecting the amount of negative feedback around the Q8 and Q9 stage,the profile can be adjusted to be mostly odd or mostly even harmonics.Q8 and Q9 are selected for non-linearity in their I/V curve with a curvetracer. Transistor Q9 can be replaced with a bipolar transistor in orderto get a different distortion profile. The control VR1 is used toprogressively increase the amount of even harmonics by progressivelyloading the collector of Q8.

Q3 and Q11 form a linearized emitter follower whose purpose is toconvert high input impedance to low output impedance. The linearizationis provided by constant current source Q11 and associated components.

An overall negative feedback at R10 and R13 is used to control theoverall gain of the complete circuit and is coupled to the source of Q9by emitter follower Q4. Emitter follower Q4 is providing isolationbetween input and output circuits.

With this non-linear amplifier circuit, it is possible to adjust boththe overall amplitude of the distortion and almost individually the evenand odd orders of harmonic distortions to achieve an overall balancedharmonic spectrum.

FIG. 8 is also a schematic diagram of a digital audio playback apparatusincluding an audio signal processor that creates and combines a desiredorder of harmonic distortion to the audio track—in this case a secondorder harmonic distortion. This apparatus is not limited to 2nd ordersof harmonic distortion but can be used for any order of harmonicdistortion. This can be used for mastering the original media file topre-compensate for harmonic distortion deficiencies in typical playbackequipment or to improve the actual harmonic distortion spectrum in themaster media file.

A digital performance embodiment of the analogue circuit described inFIG. 3 using a Digital Signal Processor (DSP) can also achieve similardistortion spectrums as identified in FIGS. 4A,B,C & D, 5A & B and 6A inthe balancing of the overall spectrum. With a DSP, a second orderharmonic signal can be generated and then added to the original track.This can be achieved by processing the original track to obtain a FFTdataset which can then be shifted to create the second order harmonic.The amplitude of the dataset can be attenuated to correspond to thedesired second order harmonic requirement. In order to combine theoriginal track with the second order harmonic of distortion track, theFFT dataset can be converted using an inverse transform to a time trackand then mixed with the original track. The resultant track can beeither stored in its modified digital form for subsequent playback orplayed in real-time without any long-term storage. The resultantmodified track now contains second order harmonic distortion. Any othersignificant orders of harmonic distortions similar to those identifiedin FIGS. 4A,B,C & D, 5A & B and 6A can be added to balance the overallharmonic spectrum.

In either the analogue or digital domain, this technique can be used tocorrect unbalanced orders of harmonic distortion associated with varioustechnologies involved at the time of the original master recording. Thenew remastered copy with balanced orders of harmonic distortion willsound subjectively less fatiguing and more intelligible.

In some embodiments, the improved audio amplifier can be used in ahearing aid. A hearing aid microphone provides the audio signal sourceand the amplified audio signal with the correct balance of THD isprovided to the output audio transducer of the hearing aid to providefor less fatiguing and more intelligible hearing enhancement.

1. (canceled)
 2. A method of audio signal reproduction comprising:providing a source audio signal from an audio signal source; providingat least one amplifier stage for amplifying said source audio signal toproduce an audio output signal; and feeding said audio output signal toan audio transducer for listening purposes, wherein a composition oftotal harmonic distortion (THD) in said audio output signal is changedto improve a listening quality, wherein said providing at least oneamplifier stage comprises providing a loaded transistor amplifiercircuit including an output load that causes greater second orderharmonic distortion energy than third order harmonic distortion energyto be produced in said loaded transistor amplifier circuit.
 3. Themethod as defined in claim 2, wherein said loaded transistor amplifiercircuit produces said second order harmonic distortion energy more than5 dB greater than third order harmonic distortion energy.
 4. The methodas defined in claim 2, wherein said load is adjusted until said secondorder harmonic distortion energy is greater than said third orderharmonic distortion energy in said output audio signal.
 5. A method ofaudio signal reproduction comprising: providing a source audio signalfrom an audio signal source; providing at least one amplifier stage foramplifying said source audio signal to produce an audio output signal;and feeding said audio output signal to an audio transducer forlistening purposes, wherein a composition of total harmonic distortion(THD) in said audio output signal is changed to improve a listeningquality by said at least one amplifier stage using a loaded transistoramplifier circuit including a load that causes greater second orderharmonic distortion energy than third order harmonic distortion energyto be produced in said loaded transistor amplifier circuit, said loadbeing adjusted until said second order harmonic distortion energy isgreater than said third order harmonic distortion energy in said outputaudio signal, wherein said load of said input stage comprises atransistor string to reduce an amount of harmonic distortion by bendingof the transfer function of said transistor amplifier circuit, and saidload is adjusted by selecting a number of transistors in said string. 6.The method as defined in claim 2, wherein said amplifying furthercomprises amplifying said source audio signal using a solid-stateamplifier circuit that produces lower odd order harmonic distortionenergy than even order harmonic distortion energy, wherein saidcomposition of total harmonic distortion (THD) in said audio outputsignal has a balance of odd and even order harmonic distortion energythat includes greater second order harmonic distortion energy than thirdorder harmonic distortion energy.
 7. The method as defined in claim 6,wherein said solid-state amplifier circuit provides power amplification,while said loaded transistor amplifier circuit providespre-amplification.
 8. The method as defined in claim 2, wherein saidloaded transistor amplifier circuit is a negative feedback amplifiercircuit whose amount of negative feedback is selected to change therelative amount of second order and third order harmonic distortionenergy in said audio output signal.
 9. The method as defined in claim 2,wherein said output audio signal has a fourth order harmonic distortionenergy less than said third order harmonic distortion energy.
 10. Themethod as defined in claim 2, wherein said output audio signal has fifthand higher orders of harmonic distortion energy that are imperceptibleto the average adult human hearing.
 11. The method as defined in claim15, wherein said at least one amplifier stage comprises a transistorcircuit generating negligible second and fourth order harmonicdistortion energy and non-negligible third and fifth order harmonicdistortion energy to amplify said mixed soundtrack.
 12. The method asdefined in claim 16, wherein said at least one amplifier stage comprisesa transistor circuit generating negligible second and fourth orderharmonic distortion energy and non-negligible third and fifth orderharmonic distortion energy to amplify said mixed soundtrack.
 13. Themethod as defined in claim 17, wherein said at least one amplifier stagecomprises a transistor circuit generating negligible second and fourthorder harmonic distortion energy and non-negligible third and fifthorder harmonic distortion energy to amplify said mixed soundtrack. 14.The method as defined in claim 2, wherein said providing said sourceaudio signal comprises providing a hearing aid microphone, said audiotransducer is an output audio transducer of said hearing aid.
 15. Amethod of audio signal reproduction comprising: providing a source audiosignal from an audio signal source as an input soundtrack; producing asecond order harmonic copy of said input soundtrack using a Fouriertransform and shifting a frequency of the input soundtrack to be adouble of the original to simulate second order harmonic distortion;mixing said input soundtrack with said second order harmonic copy toproduce a simulation of second order harmonic distortion energy withinsaid input soundtrack in a mixed soundtrack; providing at least oneamplifier stage for amplifying said source audio signal to produce anaudio output signal; and feeding said audio output signal to an audiotransducer for listening purposes, wherein a composition of totalharmonic distortion (THD) in said audio output signal is changed toimprove a listening quality.
 16. The method as defined in claim 15,wherein said output audio signal has a fourth order harmonic distortionenergy less than said third order harmonic distortion energy.
 17. Themethod as defined in claim 15, wherein said output audio signal hasfifth and higher orders of harmonic distortion energy that areimperceptible to the average adult human hearing.
 18. The method asdefined in claim 15, wherein said source audio signal is digital, andsaid processing said source audio signal using digital signal processingto introduce only second order harmonic distortion energy is donewithout changing a sampling rate of said source audio signal.